The present invention relates to implantable prostheses, such as structures or shaped articles to repair or replace bones, joints or hard skeletal parts. Articles of this type have traditionally been fabricated from strong or durable and biocompatible materials, using one or more manufacturing processes such as casting, forging, machining, coating and other industrial steps to form, shape and finish the article.
Precision powder injection molding (PPIM) is a recent refinement of metal injection molding (MIM) technology, a process that allows wet shaping of materials ranging from low carbon steels to intermetallic compounds, including traditional ceramics, high temperature ceramics, and refractory metals. The PPIM manufacturing technique is applicable to a wide variety of materials and uses, and has been applied to or is applicable to a large spectrum of specific manufacturing tasks, including the manufacture of tools, microelectronic packages, mechanical components of firearms, automotive components, and biomedical instruments. This technology has the potential to take MIM into a new era in terms of low manufacturing cost, achievable tolerances and component sizes, part-to-part consistency, and environmental friendliness. Presently, the process involves compounding the solid powder or powders which are the basic constituents of the finished part with a fluid or plastic carrier that enables the powdered material to flow for injection molding, and to maintain a stable shape when molded so that the intermediate part or compact can undergo further processing. The injection molding feed stock generally contains water soluble binder materials which are later dissolved out, and other, plasticized thermoset binder materials which remain, thus allowing the component to retain its shape until final sintering. These binder materials may also be selected to alter surface properties of constituent metal powders or to affect their chemistry, in addition to enhancing the flow of feed stock, the ultimate packing density of the solids and the dimensional stability of the molded green part. The powder constituents themselves may be selected with size, shape or elemental composition in a manner to increase solids density, improve alloying temperature, or otherwise improve characteristics such as strength, precision and ease of manufacture.
In the field of medical prostheses, porous coatings have previously been used on implants to improve cement fixation, and/or to provide a texture adapted for the ingrowth of new bone, so as to enhance the long-term fixation of the implanted device in position. Conventional approaches to creating and attaching a porous coating often involve locally attaching small features such as beads or powders, by techniques such as sintering, flame spraying, co-casting or welding; or they involve creating a porous region of the device by techniques such as etching or machining, or attaching a separate previously textured plate.
Cost is a major concern in the creation of such surgical components, and additional process steps such as sintering of a porous coating onto an implantable prosthesis add time and production costs to the product delivery. High temperature sintering can also degrade physical properties of the material which forms the body of the prosthesis. Moreover, the design of ingrowth geometries is limited by the powder particles utilized. Ceramic composites made by this approach use relatively large, e.g., 10-50 micron, agglomerates of polymer-coated inorganic particles. These agglomerate powders may spread into uniform layers and fuse to yield porous green parts that have relative densities near 50%, thus providing deep or internal connective passages, and having sufficient strength to be handled and shipped. This ability to create a structure of interconnected pores in a bioceramic body may be used for fostering bone growth, or for implementing hybrid constructions such as metal matrix/ceramic bodies which combine the wear propertied ceramic with the strength or toughness of metal for prosthetic implants, such as artificial hips. However, these hybrid porous structures can be difficult to manufacture.
In prior art PPIM systems, powders and binders are mixed to form the feed stock which is to be powder injection molded. The feed stock production is the most important step in the powder injection molding technology, and if components are manufactured from inferior feed stock, it will be difficult, if not impossible, to produce consistent components of high tolerance without resorting to secondary or further processes such as coining or machining. Thus, feed stock homogeneity and compositional accuracy are a major challenge for manufacturers using powder injection molding. Problems with components such as cracking and non-uniform shrinkage which arise during de-binding and sintering can often be traced to faulty feed stock formulation. However, the compounding of a feedstock is generally directed to these two major problems, which are addressed, for example by generally employing relatively small amounts of binder so that the molded article has a high density of solids, or is addressed by employing carrier portions that allow the use of smaller solid particles, thus increasing both the local uniformity and the packing density of the green article.
Conventional practice in powder injection molding is that powders having the elemental composition of the desired final product are mixed with a binder mixture and possibly various conditioners or additives to form the feedstock. The binder may be a heterogeneous mixture containing two primary components. The first component of the binder, also referred to as the major component, is typically a polymer component such as a wax or a water soluble polymer material, that provides, in part, a flowable, plastic or lubricating medium to facilitate the transport of the powder into the mold. The major component is also typically selected to provide good moldability and to be easily removed during the de-binding phase. Some suitable binder components of this type may be polymers of acrylic or methacrylic acid, acrylamide, vinyl acetate, ethylene glycol and various block polymers or copolymers. The second component of the binder, also referred to as the backbone component, is generally added in a lesser amount to provide strength or adhesion of the solids, i.e., to assure that the molded compact retains its shape after molding, while the first component is being removed and until the molded part is sintered. The backbone component may, for example, be a plasticized thermosetting organic material. This material may be removed later, just before the powder particles start to sinter, for example by baking out, by pyrolysis or catalytic breakdown, by chemically reactive removal or even by combination with the powder constituents near the sintering temperature. The result is a strong molded component, which sinters to form a finished part of precise dimension and improved strength. Further specific aspects of powder injection molding as well as suitable binder materials for feed stock formulation and other materials for coating of feed stock powders or enhancing binder behavior during the mixing, injection, mold separation or debinding procedures, are discussed in the following U.S. Patents and the texts referred to therein: U.S. Pat. Nos. 5,691,920; 5,639,402; 5,627,258; 5,531,958; and 5,421,853.
However, to applicant's knowledge, components made by precision powder injection molding are generally solid bodies of relatively dense material. The creation of texture in such a body would appear to require the use of a texture-patterned mold surface in the original molding step. The provision of such a pattern on the mold surface might prevent removal of the molded article from the mold, and even if this approach were found to be feasible, it would be useful for only a limited range of texture dimensions and shapes. Another approach to texturizing could perhaps be implemented by steps of separately attaching to or machining a texture region on an article molded by PPIM, although this would result in a more costly manufacturing process.